1. Field Of The Invention
The present invention relates generally to reaction chambers for chemical vapor deposition (CVD) and, more particularly, to an improved CVD reaction chamber characterized by optimized gas flows, improved materials utilization and a maximized deposition area.
2. The Prior Art
In the late 1950's, Ruehrwein invented a process for the production and deposition of high purity epitaxial films of large single crystals of inorganic compounds onto substrates by a high temperature vapor-phase reaction in the presence of hydrogen. See the U.S. Pat. No. 3,364,084. Since then, the epitaxial growth of compound semiconductors utilizing organometallic compounds has become widespread in high performance electronic and optoelectronic devices. Metalorganic chemical vapor deposition (MOCVD) generally refers to the deposition of multiconstituent films, whether epitaxial, polycrystalline or amorphous, employing any of several metalorganic compounds for the source of one or more of the constituents. In the epitaxial deposition field, many workers prefer to call this technology organometallic vapor-phase epitaxy (OMVPE). In all cases, the vapor-phase reaction takes place in a reactor having a heated susceptor and a thereon disposed substrate. In the reactor, the organometallic compounds are pyrolized by the heat of the susceptor and the substrate to form the respective atomic or molecular forms of the constituents, which constituents recombine to form the semiconductor films on the substrate.
Two basic types of reactor geometries have been used in the MOCVD growth of compound semiconductors: horizontal and vertical. The flow patterns and the resultant wafer growth characteristics of these two different designs differ markedly. In a horizontal reactor design, the flow is highly laminar. Such a laminar, displacement mode of operation results in well-characterized flow, temperature and reactant concentration gradients over the susceptor which, in addition to being well defined, vary systematically in the direction of the flow. Horizontal reactor designs lend themselves to abrupt changes in gas compositions, a most desired feature. FIG. 3 illustrates the gas flow patterns over a susceptor in a horizontal reactor geometry. A stagnant layer forms adjacent the susceptor where the flow rate is lower than that of the gas above this layer. The thickness of this stagnant layer increases with distance along the susceptor, with consequent depletion of the reactant species. This downstream depletion and temperature change result in corresponding concentration and thickness gradients with wafer position along the axial length of the horizontal reactor. Another stagnant zone in the horizontal reactor is created by the gases passing over the reactor tube surface. A vertical reactor, on the other hand, operates in a non-laminar, turbulent mode, precluding abrupt changes in gas compositions. A major drawback of the vertical design is its tendency to achieve non-uniform crystal growth over the entire susceptor surface. Further, the vertical design also suffers from lower throughput capability due to a small susceptor area limited by the cross section of the reaction tube. A further shortcoming of some vertical reactor designs resides in occasional prenucleation occurring in the gas phase. FIG. 4 illustrates a typical vertical reactor arrangement.